Currently, two-lens cameras have been designed by following two trends, i.e., a zoom-lens model and a small-sized model. Both of these models have become highly demanded products in the market. Generally, the zoom-lens model has been classified into two categories according to the operation of its movable elements, i.e., the optical compensation type and the mechanical compensation type. The mechanical compensation type of zoom lens tends to have a smaller size than that of the optical compensation type, and therefore it has become a popular requirement for a zoom-lens camera to have a light weight, and a thin and small size.
In U.S. Pat. Nos. 5,144,493, the pertinent parts of which are shown and 5,144,493 as shown in FIG.1, it is illustrated a conventional mechanical compensation type of zoom-lens, which comprises a lens mount assembly 10 having a first lens mount 11, and a second lens mount 12, a straight-slot cylinder 13, an inner cylinder 14, and an outer cylinder 15. In other words, the zoom-lens includes four cylinders. The inner cylinder 14 is provided with a plurality of curved slots 140 to provide a variable distance between the first lens mount 11 and the second lens mount 12, so as to keep a focal plane at a desired position. The straight-slot cylinder 13 is used for limiting the optical parts in the lens mount assembly 10 to move back and forth along a straight line.
In the aforesaid mechanical compensation type of zoom-lens, the first and second lens mounts 11 and 12 of the lens mount assembly 10 have to move back and forth along a straight line. Therefore, both the straight-slot cylinder 13 and the inner cylinder 14, are required. As a result, it is impossible to minimize the diameter of the lens is unable to minimize to a break-through level because of the requirement of having too many cylinders therein.